Effect of Dent Depth in Ultrasonic Spot Welding Joints used Extruded Mg96Zn2Y2 Alloys

An extruded Mg96Zn2Y2 alloy has excellent mechanical properties and a flame-retardant. The alloys have two phases of an Mg matrix and a long-period stacking ordered structure phase (LPSO phase) extended along the extrusion direction. By contrast, the ultrasonic welding technology is a solid-state bonding and the welding is achieved by the severe plastic deformation in the welding processes. However, the dents generated during the welding process are often taken up as issues that affect the weldability. This paper evaluated the relationship between the dent depth and the microstructure in the joints when the welding conditions are kept constant. After welding, the plastic flow due to the folding phenomenon was observed. When the dent depths became excessive and the sheet thickness reduction rate increased, the LPSO phases extending in the extrusion direction was not maintained. And, the morphology changed into mottled and plate-like LPSO phases. The continuity of the originally LPSO phases was lost, and the plastic deformation progressed during the welding process. It was considered that the heat generated during the process and the severe plastic deformation were the cause. And then, in the joints with the largest sheet thickness reduction rate, a molten structure was formed.